const singer = [
    { name: '周杰伦', num: 20 },
    { name: '许嵩', num: 25 },
    { name: '林俊杰', num: 19 },
    { name: '蔡徐坤', num: 10 },
    { name: '鹿晗', num: 0 },
  ]
  
  Array.prototype.my_find = function(callback) {
    for (let i = 0; i < this.length; i++) {
      if (callback(this[i], i, this)) {
        return this[i]
      }
    }
    return undefined
  }
  
  const res = singer.my_find((item, index, arr) => {
    return item.num > 20
  })
  
  console.log(res);